The present preferred embodiment relates to a device and a method for electrically charging a transport belt for the transport of recording media in the transfer printing area of an electrographic printer or copier. Further, the preferred embodiment relates to an associated contact element.
In electrographic printers or copying devices, the transfer of a toner image from an intermediate carrier, for example a photoconductor drum or a photoconductor belt, onto a recording medium is referred to as transfer printing. The section of the printer or copying device in which the intermediate carrier and the recording medium are brought into contact is referred to as the transfer printing area. In the transfer printing area, the intermediate carrier, for example the circumferential surface of a photoconductor drum, and the recording medium, move at the same speed in the same direction, while the toner is transferred from the intermediate carrier onto the recording medium.
A good print image on the recording medium can only be achieved if a uniform contact is established between the recording medium and the intermediate carrier in the transfer printing area. A good and uniform contact between the recording medium and the intermediate carrier can be achieved with the aid of an electrostatically chargeable transport belt on which the recording medium is transported through the transfer printing area in a manner that it rests on the transport belt and adheres thereto as a result of electrostatic forces.
A device for transferring a toner image with the aid of an electrostatically chargeable transport belt is illustrated in the document DE 102 47 368 A1, which is herewith incorporated by reference into the present application. In this device, the transport belt is charged with a charge whose sign is opposite to the sign of the charge of the toner image. This electrostatic charging of the transport belt has a double function: on the one hand, it results in an electrostatic attraction of the recording medium to the transport belt and thus in a safe guidance of the recording medium in the transfer printing area, and on the other hand, it causes the transfer of the toner image from the intermediate carrier onto the recording medium.
Similar devices having electrostatically charged transport belts are also known from the documents U.S. Pat. No. 5,666,622, DE 195 01 544 A1 and U.S. Pat. No. 5,159,392. In these three documents, the transport belt is charged either by means of corona arrangements, so-called corotrons, or by means of contact rollers. A corotron usually comprises one or several thin gold-coated tungsten wires, the electric potential of which with respect to a grounded housing amounts to several 1000 V so that the air surrounding these wires is ionized.
However, corotrons have a number of serious disadvantages, for example, the ozone formation as a result of the high charging voltage and the relatively complex replacement of worn corotron wires. In addition, the corotron wires are easily contaminated with dust, wear particles from the belt and toner particles, this resulting in an irregular charge distribution on the transport belt. Areas having a lower transport belt charge result in a less complete transfer of the toner onto the recording medium resting on the transport belt and thus in an undesired brighter print image. The cleaning of the corotron wires is not only complex but also exerts a considerable mechanical stress on these wires and shortens their life.
Contact rollers, too, have the disadvantage that they can easily be contaminated and thus cause an irregular charging of the transport belt. Furthermore, they cannot be directly arranged in the transfer printing area since they would disturb the uniform bearing of the recording medium against the intermediate carrier. In order to achieve that the transport belt is nevertheless sufficiently charged in the transfer printing area, a certain amount of current has to flow from the point of contact between the contact roller and the transport belt to the transfer printing area. Therefore, the conductivity of the transport belt must not be too low, which is a disadvantageous restriction on the choice of the transport belt material used.
From the documents DE 102 47 368 A1 and WO 2004/046830 A1, a device for electrically charging a transport belt is known, which device comprises a blade-like contact element comprising a plastic film, and in particular a polyimide film. Soot particles are embedded in the plastic film for lowering the conductivity of the film to a desired value.
The known contact element made of plastic film can be directly arranged in the transfer printing area on the side of the transport belt facing away from the intermediate carrier and thus provides a reliable uniform charging of the transport belt in the transfer printing area. Since the transport belt continuously rubs against the contact element, the same is constantly cleaned.
However, this known blade-like contact element has disadvantages, too. On the one hand, the known blade-like contact elements are relatively expensive to produce. Their specific conductivity must be exactly adapted to the transfer printing system used so that good charging and transfer printing results can be achieved. This means that the plastic films must be specifically produced which is only economic for larger batches which are not required. In addition, it turned out to be difficult to establish a truly homogeneous distribution of the conductivity within the film, this having negative effects on the transfer printing.
Further, the known blade-like contact elements have to be installed very precisely, in particular with a very narrow fitting tolerance with respect to the distance to the transport belt. If the transport belt runs horizontally in the transfer printing area, as is usual, this means that the known blade-like contact element has to be installed with a very tight vertical tolerance, which is complicated both when the known contact element is installed as well as when the same is maintained or replaced and causes considerable costs. If the known contact element is installed at too great a distance from the transport belt, there results an uncertain electrical contact between the contact element and the transport belt. However, if the known contact element is installed too close to the transport belt, it is bent upon contact with the transport belt and thus exerts a pressure force on the transport belt. However, an increased pressure force is not desired since it results in a so-called mechanical transfer printing in which a toner background present on the photoconductor is transfer printed onto the recording medium by the mechanical pressure force.